The present invention is directed to a micro mirror structure, and more particularly, to a micro mirror structure with high reflectivity and flatness.
Micro mirror structures, such as micro mirror arrays, are typically used in an optical cross connect engines or other devices or applications to reflect and route optical signals. A mirror array may include a plurality of individually movable mirrors or reflective surfaces which can reflect and redirect an incoming signal in a desired direction. Each individual mirror in the mirror array may include a substrate and a thin metal film located on the substrate to enhance the reflectivity of the mirror. Each mirror is preferably relatively thin so that: 1) the mirror has a high resonant frequency that is outside of environmental vibration frequencies; 2) the mirror is light and can react quickly to actuation forces to achieve large deflection angles; and 3) the mirror is about the same thickness as the springs, which allows easy processing.
Each mirror should have high flatness and high reflectivity to ensure signals reflected by the mirror are accurately directed with a minimum loss of the strength of the signal. However, when a reflective metal is located on a substrate, the reflective metal may diffuse through to the substrate, particularly when exposed to elevated temperatures, which can reduce the reflectivity of the mirror and the mirror""s long-term stability.
Furthermore, internal stresses in the substrate and/or metal film may induce a curvature in the mirrors, which can cause focal aberrations and astigmatisms. Processing and manufacturing of the mirror array, such as reactive ion etching processes or sputtering with use of a shadow mask, can induce further curvature in the mirrors. Furthermore, the thermal coefficient of expansion of the metal film may differ from the thermal coefficient of expansion of the substrate. Accordingly, when the ambient temperature of the mirror array increases, such as during operation of an optical cross-connect engine, the curvature in each mirror may increase due to the differing thermal coefficients of thermal expansion between adjacent materials or layers.
In one embodiment, the present invention is a mirror structure which has a substrate and reflective coating located thereon. The mirror structure is concave at a predetermined temperature and convex at another predetermined temperature such that the mirror structure can be maintained within a range of flatness over a range of temperatures. In one embodiment, a corrective layer is located above the substrate, the corrective layer being in a state of tension, and a top reflective layer is located above the corrective layer. The corrective layer may act as a diffusion barrier as well as providing stress balance to the metal film. In another embodiment, the mirror structure includes a diffusion barrier located between the substrate and the reflective layer.
In one embodiment, the invention is a micro mirror structure including a plurality of individually movable mirrors. Each mirror has a generally concave shape at a temperature of about 20 degrees Celsius and has a generally convex shape at a temperature of about 85 degrees Celsius. In another embodiment, the invention is a micro mirror structure including a plurality of individually movable mirrors arranged in an array. Each mirror may have a substrate, a corrective layer located above the substrate, the corrective layer being in tension, and a top reflective layer located above the corrective layer.
Other objects and advantages of the present invention will be apparent from the following description and the accompanying drawings.